This invention relates to a system for reducing the formation of spots when viewing images which are uniformly illuminated by means of laser projection and which are reflected off a projection surface.
Laser projection uses a raster procedure to produce images. In the raster procedure, three primary colors red, green, and blue are emitted from a laser. The three colors are modulated separately, brought together again, and scanned over a surface. The image is then formed by points, as in the standard television picture tube. The color of an image point is defined by the relative power of its primary color components. Normally the beam is deflected horizontally by a polygonal mirror and vertically by a galvanometric scanner.
The laser beam causes image irregularities (spots) or so-called speckles in the laser image projection due to the spatial coherence of the laser light. The effect can be observed by shining a diverged laser beam onto a wall. The eye of a viewer sees a spot with a stochastic light-dark distribution, instead of a uniformly illuminated field.
The spots are perceived when light is reflected off a projection surface and to the eye. The projection surface has areas which are uneven compared with the size of the light source wavelength. The light reflected off these uneven surface areas of the projection surface reach the eye with different phase positions creating interference that is perceived in the eye. From a given standpoint of the observer, this interference produces a spatial two-dimensional light-dark pattern.
A laser projection system is described in European Patent No. EP0589179A1 where laser radiation consisting of three primary colors is passed through a diffusion element in a common optical path. The diffusion element rotates in the range from 30 to 40 rpm to prevent the formation of spots due to interference phenomena and creates a clearer, truer-color image.
A projection light image display system with reduced spot formation is also described in European Patent No. EP0385706. A coherent light source is used to produce a light beam. A light modulator directs the light from the source onto the display screen reproducing an image. The screen is coupled with a transducer that creates acoustic surface waves in the screen where the image is reproduced. The acoustic waves possess an amplitude which is greater than the wavelength of the light beam. The surface waves are suppose to prevent the formation of spots when viewing uniform image contents taking advantage of the fact that the eye is not quick enough to perceive the moving interference pattern created by the surface waves.
A system is also described in U.S. Pat. No. 5,313,479, in which a rotating diffusion element is arranged in the optical path of the laser. The diffusion element moves the interference pattern so quickly that it cannot be perceived by the human eye.
The systems described above require mechanical transducers and, therefore, are relatively expensive.
Thus, a need exists for a less complex and expensive system for reducing interference in light image display systems.